An IGBT (Insulated Gate Bi-polar Transistor) is a switch component widely used in power electronic applications due to its good properties. The component can be used at high voltage levels and it can safely turn high currents on and off.
One of the properties of an IGBT component is its ability to shut down the current also in a short circuit situation. In a short circuit, all of the source voltage remains across the switch component. In an exemplary use, one or more IGBT components are used to switch voltages to a load which may be, for example, an electrical motor.
A short circuit current in the IGBT component can be the result of a short circuit in the load. In such a case, the switched high voltage does not meet any impedance and current rises rapidly through the component. In the case of a voltage source inverter, the short circuit current may begin due to shoot-through current.
The IGBT component limits the short circuit current to a component specific value which depends on the gate voltage applied to the gate of the component. This operation point is referred to as an active region or a desaturation region. When an IGBT component is in its active region, the current of the component follows the voltage applied to its gate. The current can be shut down by bringing the gate voltage to and below the threshold voltage of the component.
The shutting down of the short-circuit current can involve a very high current change rate (di/dt). The high rate of change can cause a high voltage spike in the stray inductances of the short circuit current path. This voltage spike makes the voltage across the IGBT component higher than the tolerable voltage and, therefore, the component can be destroyed.
For the above reason, a procedure known as soft turn-off is applied when short circuit current is to be shut down. In soft turn-off, the stored charges from the component are discharged more slowly than in a normal turn-off. This can be carried out by using a gate resistor that has a higher resistance than in normal shut-down operation or by using shut-down voltage that is lower than normal shut-down voltage (i.e. the potential difference is not as high as in usual operation). Also, when the component is controlled with current, a smaller shut-down current is used.
A drawback of the additional gate resistor is the separate parallel branch used in the gate driver circuitry, which can make the structure of the driver more complex. When a different control voltage is used for shut-down operation, a separate voltage level should be structured between turn-on and turn-off voltage levels.
A common drawback of the above solutions is the additional control signal that is led through the floating potential boundary. Control signals for IGBT components can be produced at the potential of a control circuitry which is different from the emitter potential of the IGBT, which is the potential of the gate control. The potentials can be separated by using optoisolators, and the signal for the soft shut-down doubles the number of these components.